The present invention relates to a method for boron doping wafers using a vertical oven system. The boron doping of wafers, in particular silicon wafers, plays an important role in semiconductor technology. The present method can be used in particular for the production of semiconductor products such as power MOSFETs (metal oxide semiconductor field effect transistors) in the DMOS technology (double diffused metal oxide semiconductor technology) or for bipolar transistors.
Two different, technologically relevant techniques have been used so far for the boron doping of silicon wafers. One technique concerns the direct boron implantation into the silicon wafer, whereas the other technique uses source layers for diffusing the boron into the silicon wafer.
The first-mentioned boron-implanting method, for which boron ions are accelerated and impact at high speed with the silicon wafer, however, results in extremely high processing costs due to the high implantation dose required for specific applications. This method can furthermore be realized only as a single-wafer process, which increases the time expenditure and thus also the process costs. Another disadvantage of this technique is that the boron-concentration profile generated through implantation into the silicon wafer is not box-shaped, but has a Gaussian1 shape. A second implantation is required to achieve an erfc2 profile with this technique. However, box-shaped doping profiles in particular are needed for the aforementioned power MOSFETs and for bipolar transistors.
Thus, the second doping method mentioned herein, which makes use of source layers, is generally used for the production of doping profiles of this type at a reasonable cost. With this method, the boron doping occurs from a solid layer that is deposited on the wafer. The doping method requires a two-stage process. A thin, highly concentrated boron layer is generated in a first stage through low temperature depositing on the wafer surface. With the aid of a high-temperature diffusion process, the boron then diffuses from this thin boron layer during a second stage into the surface of the wafer, up to the desired depth.
The problem of doping uniformity and reproducibility of this doping between individual processing cycles comes becomes important as a result of the constantly increasing wafer size and the requirement for doping the highest possible number of wafers during a processing cycle. On the one hand, it must therefore be ensured that the desired doping profile on the individual wafer has the highest possible uniformity. On the other hand, the deviation in the doping profile or the doping concentration between individual wafers of a processing cycle, as well as between wafers from different processing cycles, must be negligibly small.
One variant of the method for boron doping silicon wafers from a solid layer uses boron nitride wafers as boron source for generating the source layers on the silicon wafers. A method of this type is known, for example, from J. Monkowski et al., xe2x80x9cSolid State Technology,xe2x80x9d November 1976, pages 38 to 42. This method uses a horizontal oven system, in which the individual wafers are arranged one behind the other inside the so-called quartz boot. The boron nitride wafers are arranged for this between the individual silicon wafers.
The disadvantage of this method, however, is that the oven capacity for the silicon wafers that must actually be doped is reduced by 50% due to the required arrangement of the boron-nitride wafers. Furthermore, the danger exists that the quartz component(s) of the processing chamber is (are) contaminated or damaged because the boron-nitride wafer adheres to the quartz boot. Another disadvantage is the involved storage and conditioning of the boron-nitride wafers, which additionally are very costly and have only a limited durability.
Another method for boron doping silicon wafers from a solid layer is disclosed in the Reference P. C. Parekh et al., xe2x80x9cProceedings of the IEEE,xe2x80x9d Volume 57, Number 9, from Sep. 9, 1969, pages 1507 to 1512. With this method, liquid BBr3 (boron tribromide) is used as a source. Oxygen and BBr3 are fed jointly with nitrogen as carrier gas into the reaction room containing the wafers. Inside the reaction room, the BBr3 together with the oxygen forms the so-called reactive gas, which reacts as follows:
2BBr3(g)+3/2O2(g)xe2x86x92B2O3+3Br2(g)
2B2O3+3Sixe2x86x924B++3SiO2(borosilicate glass)
The borosilicate glass is thus deposited on the surface of the wafer. The borosilicate glass created in this way functions as source layer, from which boron is diffused during the subsequent diffusion phase (drive in) into the wafer substrate underneath. A horizontal oven system with an expanded, constant temperature zone was used for this method. The borosilicate glass is deposited at a temperature range between 860 and 950xc2x0 C., the diffusion occurs at 1220xc2x0 C. The problem of a uniform doping was again of the utmost importance.
One disadvantage of the method shown herein is that the doping uniformity again could not be maintained, in particular over the length of the horizontal oven used.
Furthermore, larger wafer diameters cannot necessarily be processed automatically when using a horizontal oven system. Thus, maximum 5-inch wafer diameters can presently be doped inside horizontal oven systems. A change from the 5-inch wafer to the 6-inch wafer is possible only with great difficulties because of the required change in the processing specifications with respect to the doping uniformity of the silicon wafer.
Starting with this state of the technology, it is the object of the invention to specify a method for boron doping wafers, which makes it possible to achieve a high doping uniformity without requiring structural changes in the existing oven systems when changing from smaller to larger wafer diameters. In addition, a cost-effective realization of this method should be possible.
This object is solved with the method according to claim 1. Advantageous embodiments of the method are the subject matter of the dependent claims.
A vertical diffusion oven is used with the method according to the invention for boron doping wafers. This oven is provided with a vertical reaction chamber, extending from an upper end toward a lower end, which comprises several independently heated temperature zones. A gas intake for a boron-containing reactive gas is located at the upper end of the reaction chamber. The individual temperature zones extend successively from the upper end toward the lower end of the reaction chamber. With the method according to the invention, the boron-containing reactive gas flows over the wafers arranged inside the reaction chamber to deposit a layer of boron, in particular a layer of borosilicate glass. Subsequently, the boron from the boron layer is diffused into the surface of the wafer. According to the invention, the temperature in the independently heated temperature zones is adjusted such that between the zone following the top temperature zone and the lowest temperature zone, a temperature increase is maintained during the deposit of the boron layer and a temperature drop is maintained during the subsequent diffusion.
Inside the reaction chamber of the vertical oven, these additional temperature zones extend over the region filled with wafers. The upper zone covers the region of the gas intake. The temperature increase or the temperature drop toward the lower end of the reaction chamber is initiated through a stage-by-stage increase or reduction in the temperature from zone to zone. Excellent results can be obtained with a vertical diffusion oven that is divided into five temperature zones, wherein the middle temperature zone extends over approximately half the height of the reaction chamber. The boron-containing reactive gas can be provided through different, liquid or gas-containing boron sources. Examples for these are BBr3, BCl3 or B2H6 sources.
The method according to the invention on the one hand differs from the aforementioned methods in that a vertical oven is used and, on the other hand, in that different temperature zones with different temperatures are maintained during the depositing and the diffusion processes.
With the method according to the invention and using the vertical oven system, it is possible to transfer a boron doping process from silicon wafers with a maximum diameter of 5 inches, which is standard for the aforementioned prior art, to 6 inches without requiring involved structural changes on the diffusion oven. The method makes it possible to achieve an improved doping uniformity, in particular relative to the aforementioned methods, with respect to the uniformity of the layer resistance across the wafer as well as across the total reaction chamber or the length of the quartz boot used. The reproducibility of the results from process cycle to process cycle is furthermore excellent.
The method according to the invention avoids the use as well as the maintenance of expensive boron nitride source wafers and permits the doping of a silicon substrate with high-concentration boron in the range above 1xc3x971019 cmxe2x88x923 on the wafer surface. The dopant in this case has a box-shaped concentration profile that is approximately described with an erfc function. As a result, a stable and economic doping process is available for the special requirements of power MOS and bipolar semiconductor processes with respect to the form of the boron concentration profile.
The temperatures for depositing the boron layer are preferably selected from the range between 800xc2x0 C. and 950xc2x0 C. and those for the diffusion from the temperature range between 1020xc2x0 C. and 1050xc2x0 C. Good results are obtained in particular if the temperature in the upper zone for the deposit as well as the diffusion is selected higher than the temperature of the respectively following temperature zone. This temperature of the upper temperature zone influences the temperature of the reactive gas that flows in through the gas intake.
The reactive gas is preferably made available via a BBr3 bottle (BBr3 bubbler) by mixing it with oxygen.
When using a vertical diffusion oven with at least five temperature zones, excellent results can be achieved with the method according to the invention if the temperatures for depositing the boron layer from the upper to the lowest zone are set to 860xc2x0 C., 845xc2x0 C., 860xc2x0 C., 890xc2x0 C. and 900xc2x0 C., respectively with an accuracy of xc2x15xc2x0 C., and for the diffusion to 1042xc2x0 C., 1037xc2x0 C., 1035xc2x0 C., 1027.5xc2x0 C. and 1025xc2x0 C., respectivity with an accuracy of xc2x10.5xc2x0 C. These temperature settings result in a high uniformity of the introduced doping profiles.
When using a mixture of oxygen and BBr3 as reactive gas and nitrogen as carrier gas, particularly advantageous results can be achieved, given a reaction chamber volume of 50xc2x15 liters, with gas flows of 10 slmxc2x10.5 slm for the carrier gas, 0.1 slmxc2x10.01 slm for the oxygen and 0.1 slmxc2x10.01 slm for the BBr3. The person skilled in the art can adapt these values to a reaction chamber with a different volume.
Ways to Realize the Invention
The invention is explained once more in the following with the aid of an exemplary embodiment and in connection with the drawing, without restricting the general inventive idea.